The present invention relates to a magnetic recording medium, which comprises a lower layer and one or more magnetic layers, said lower layer having at least one of magnetic powder or non-magnetic powder and a binder dispersed therein and being disposed on a non-magnetic support member, said magnetic layer having ferromagnetic powder and a binder dispersed therein, and said medium having good electromagnetic transfer characteristics and high durability.
The magnetic recording medium is widely used in such applications as tape for recording, video tape or floppy disk. In a magnetic recording medium, a magnetic layer having ferromagnetic powder dispersed in a binder is laminated on a non-magnetic support member.
The magnetic recording medium must be at high level in properties such as electromagnetic transfer characteristics, running durability, and running performance. Specifically, an audio tape for recording and reproducing musical sound must have high ability to reproduce the original sound. A video tape must have good electromagnetic transfer characteristics such as high ability to reproduce original picture.
In addition to good electromagnetic transfer characteristics as described, the magnetic recording medium must have high running durability as described above. In order to provide good running durability, abrasive material and lubricant are generally added to the magnetic layer.
However, to provide high running durability, the quantity of the abrasive to be added must be increased, and this means decrease of filling degree of the ferromagnetic powder. In case an abrasive material having larger particle size is used to ensure good running durability, the abrasive material tends to protrude on the surface of the magnetic layer. Therefore, improvement of the running durability using the abrasive material often leads to deterioration of the electromagnetic transfer characteristics as described above.
When it is attempted to improve the running durability using a lubricant, the quantity of the lubricant to be added must be increased. As a result, the binder is very likely to be plasticized, and the durability of the magnetic layer tends to decrease.
In the improvement of the durability and the electromagnetic transfer characteristics, the binder, serving as one of major components of the magnetic layer, plays an important role. Vinyl chloride resin, cellulose resin, urethane resin, acrylic resin, etc. used for this purpose in the past have problems in that abrasion resistance of the magnetic layer is deteriorated and running components of the magnetic tape are contaminated.
Contamination of magnetic head causes deterioration of electromagnetic transfer characteristics. In particular, in equipment and devices used for high density recording, number of revolutions of the magnetic head is increased. The number of revolutions of the magnetic head is 9,600 rpm in digital video tape recorder, and this is far higher than 1,800 rpm in analog video tape recorder for home use and 5,000 rpm in video tape recorder for business use. As a result, sliding speed between the magnetic recording medium and the magnetic head increases. Also, small size magnetic head such as thin film head is now used. In this respect, there are now strong demands on improvement or elimination of contamination of the magnetic head caused by the substances generated from the magnetic recording medium.
To solve the above problems, several methods have been proposed to increase hardness of the magnetic layer using hard binder.
For example, JP-A-3-88119, JP-A-3-83221, or JP-A-3-216812 disclose as follows: In a magnetic recording medium having an upper magnetic layer and a lower coating layer, a polyester polyurethane resin is used as a binder, and a substance having glass transition temperature (Tg) of 40.degree. C. to 100.degree. C. or a substance having high dynamic strength such as aromatic polyurethane is used as a binder for the upper magnetic layer. More concretely, a substance such as polyester urethane is described. These substances having aromatic compounds have problems in that they cause the decrease of solubility in a solvent and reduces dispersion property. It is described that a substance having Tg of -10.degree. C. to -50.degree. C. or aliphatic polyurethane such as polyester, polyether, etc. can be used as the material for the lower layer, but these have very low dynamic strength and are insufficient in ability to adsorb to powder. As a result, these are not satisfactory in dispersion property. Also, these substances are disadvantageous in that the lower layer polyurethane tends to deposit on the surface of coating film in coating film drying process and that dynamic strength of coating film surface is low and durability is not satisfactory.
As the material for the lower layer, JP-A-7-176042 discloses the use of polyurethane, which has such structure that 1 to 2 mols of propylene oxide (PO) are added to bisphenol A, but this is unsatisfactory in ability to adsorb to powder and also in dispersion property. If PO-added structure is increased in bisphenol A in order to improve adsorbing ability, the quantity adsorbed to powder is increased, while solubility in a solvent is decreased because bisphenol A skeletons having aromatic compound increase. JP-A-1-268729 discloses in the example the use of polypropylene oxide (PPO) added to bisphenol A as a polyether urethane having cyclic structure for a single layer magnetic layer, while PPO component shows low dynamic strength of coating film and it is not satisfactory in durability.
Further, JP-A-7-50010 discloses a magnetic recording medium using a polyurethane, which contains a long-chain diol by 0 to 5 mol %. However, coating film is too hard because it mostly consists of hard segments and has low moldability in calender process. Further, the coating film is not sufficiently smooth and electromagnetic transfer characteristics are also not satisfactory.
As described above, conventional type polyurethane resin or polyurethane urea resin used as a binder for the magnetic recording medium are not very satisfactory in terms of durability.
To solve the above problems, it is an object of the present invention to provide a magnetic recording medium, which has high calender moldability, provides very smooth coating film surface and shows good electromagnetic transfer characteristics, and further has coating film surface of high dynamic strength and good running durability.